If you're ever unsure whether you can regen-fight a monster or not and want to avoid wasting blackspace, you can find out by calculating the attritions. Attrition = damage done to opponent (just read it from the predictor to skip resistance calculations) * your regeneration (usually the level of the character, unless you have double regen).
If your attrition is higher than your opponent's you can regen-fight it. When measuring attritions what matters is their ratio not the difference. The higher the ratio, the more efficient it will be, so you will be done with less blackspace consumed.
There are a plethora of ways to increase your attrition: increasing resistances, damage, regeneration, using a damage or healing glyph... Leveling up is really important as it boosts both damage and regeneration, so if you've decide to kill your boss with this method the closer you are to lvl 10 the better it is.
Just to mention regen-fighting is not unique to Monks any character with good resistances or just insane damage can do it. If you ever notice that your lvl 10 hero has more damage than your boss, it's a dead giveaway you can regen-fight it. The best thing about regen-fighting a boss (or any other monster) is that so long you have blackpsace you caan ignore the health of the target. Other fighting methods cannot do this, they have a cap how much damage they can output, but not regen-fighting
